Discovery of a brown dwarf very close to the Sun: A methane-rich brown dwarf companion to the low-mass star SCR 1845-6357

We present VLT/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered M8.5 star just 3.85 pc from the Sun. Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4.5 AU (1.170 +/- 0.003 on the sky) and fainter by 3.57 +/- 0.057 mag in the 1.575 mu m SDI filter. This substellar companion has an H magnitude of 13.16(-0.26)(+0.31) (absolute H magnitude of 15.30(-0.26)(+0.31)), making it likely the brightest mid-T dwarf known. The Simultaneous Differential Imager ( SDI) consists of three narrowband filters placed around the 1.6 mu m methane absorption feature characteristic of T dwarfs (T-eff < 1200 K). The flux of the substellar companion drops by a factor of 2.7 +/- 0.1 between the SDI F1 (1.575 mu m) and F3 (1.625 mu m) filters, consistent with strong methane absorption in a substellar companion. We estimate a spectral type of T5.5 +/- 1 for the companion based on the strength of this methane break. The chances that this object is a background T dwarf are vanishingly small-and there is no isolated background T dwarf in this part of the sky, according to 2MASS. Thus, it is a bound companion, hereafter SCR 1845B. For an age range of 100 Myr to 10 Gyr and spectral type range of T4.5-T6.5, we find a mass range of (9-65)M-Jup for SCR 1845B from the Baraffe et al. COND models. SCR 1845AB is the 24th-closest stellar system to the Sun (at 3.85 pc); the only brown dwarf system closer to the Sun is the binary brown dwarf epsilon Indi Ba-Bb (at 3.626 pc). In addition, this is the first T dwarf companion discovered around a low-mass star.